Case Scenario 02

From PCR to policy: new ways of controlling zoonotic sleeping sickness

Control interventions for human African trypanosomiasis (HAT) are often implemented as a crisis response when the level of disease in people is considered unmanageable. Resources are then deployed with the aim of removing infective parasites from people (by chemotherapy) and attempting to control tsetse flies. During the intervening periods farmers and communities are left to fend for themselves despite the ongoing low level disease risk. To effectively control a zoonotic disease we need to be able to identify the animals maintaining the infectious agent and estimate the size of the reservoir harbouring the disease agent. Traditional methods using microscopy to visibly identify parasites have underestimated the extent of the animal reservoir – routinely by as much as 60%. Consequently few resources have been directed at dealing with the disease in the principal reservoir, livestock. Sleeping sickness is difficult and expensive to treat in people and treatment itself has a high risk of mortality (up to 5%). In contrast, elimination of the parasite from animal hosts is affordable and effective; a single treatment with a cheap injectable drug is sufficient to clear the animal of all circulating trypanosomes.

Modern PCR techniques are now able to detect species of trypanosome circulating in animal hosts and determine what proportion of these parasites are human infective. In areas endemic for HAT, for every three Trypanosoma brucei brucei (non-human infective) infected animals observed of any species, one of these animals will be infected with T. b. rhodesiense (human infective). Using the most sensitive PCR methods available, we have shown that in areas of East Uganda endemic for HAT, up to 85% of village cattle screened monthly over an 18-month period were positive by PCR for T. brucei and 18% of cattle harboured T. b. rhodesiense, far higher than the 1% found using traditional microscopy. Once infected, cattle, which can live for ten or more years in these production systems, maintain T. brucei infections for life, outliving tsetse control programme cycles. Since parasitaemias are relatively low in local zebu cattle and as animals rarely show visible clinical signs of trypanosomiasis, they are infrequently treated. Parasitaemias in cattle are, however, high enough for uptake by tsetse flies which only require a single trypanosome to become infected if susceptible (one parasite in 30 micro-litres of blood is sufficient). Cattle, which are critical village investments, thus present a long term health risk to rural people. As a result of this research, the Government of Uganda is tabling legislation for the block treatment of cattle around outbreaks of HAT, as well as of animals moving into new areas from HAT endemic regions, a measure which will stop transmission and thus save human lives – just under half of the people infected die without getting treatment. This will also help improve livestock productivity. Nevertheless, this type of block treatment will need to be very carefully targeted, as it is neither possible nor appropriate to repeatedly treat every animal without the risk of resistance to the drugs developing and thus compromising the efficacy of the very limited armoury of human treatments. Modern technologies need translating into affordable pen-side diagnostics so that interventions can be targeted to high-risk disease carriers and sustainable low-cost tsetse control options need to be further explored.